The number of national security letters issued by the FBI skyrocketed in the years after the Patriot Act became law in 2001, according to last year’s report by Justice Department Inspector General Glenn A. Fine. His review is required by Congress, over the objections of the Bush administration.

The FBI improperly used national security letters in 2006 to obtain personal data on Americans during terror and spy investigations, Director Robert Mueller said Wednesday.

Mueller told the Senate Judiciary Committee that the privacy breach by FBI agents and lawyers occurred a year before the bureau enacted sweeping new reforms to prevent future lapses.

Details on the abuses will be outlined in the coming days in a report by the Justice Department’s inspector general.

The report is a follow-up to an audit by the inspector general a year ago that found the FBI demanded personal data on people from banks, telephone and Internet providers and credit bureaus without official authorization and in non-emergency circumstances between 2003 and 2005.

Mueller, noting senators’ concerns about Americans’ civil and privacy rights, said the new report “will identify issues similar to those in the report issued last March.” The similarities, he said, are because the time period of the two studies “predates the reforms we now have in place.”